Add Claude Fable 5.1, Gemini 3.8 Flash

A user requested adding Claude Fable 5.1 and Gemini 3.8 Flash to an assistant, citing frequent use of Gemini 3.7 Flash and occasional use of Fable, and expressing a desire for stronger models.

Mateo-Grgic these are some new models that have come out recently, they would be useful to have in the assistant. I use gemini 3.7 flash a lot, i think it'd be nice to have 3.8 flash too. I use fable as well from time to time, same thing—it'd be nice to have a stronger model.